Conventional turbomolecular vacuum pumps include a housing having an inlet port, an interior chamber containing a plurality of axial pumping stages and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high rotational speed by a motor to pump gas between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial pumping stages.
Variations of the conventional turbomolecular vacuum pump, often referred to as hybrid vacuum pumps, have been disclosed in the prior art. In one prior art configuration, one or more of the axial pumping stages are replaced with molecular drag stages, which form a molecular drag compressor. This configuration is disclosed in U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 and assigned to Varian, Inc. sells hybrid vacuum pumps including an axial turbomolecular compressor and a molecular drag compressor in a common housing. Molecular drag stages and regenerative stages for hybrid vacuum pumps are disclosed in Varian, Inc. owned U.S. Pat. No. 5,358,373, issued Oct. 25, 1994. Other hybrid vacuum pumps are disclosed in U.S. Pat. No. 5,221,179 issued Jun. 22, 1993; U.S. Pat. No. 5,848,873, issued Dec. 15, 1998 and U.S. Pat. No. 6,135,709, issued Oct. 24, 2000. Improved impeller configurations for hybrid vacuum pumps are disclosed in Varian, Inc.'s owned U.S. Pat. No. 6,607,351, issued Aug. 19, 2003.
Molecular drag stages include a rotating disk, or impeller, and a stator. The stator defines a tangential flow channel and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. The momentum of the rotating disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet. Molecular drag stages were developed for molecular flow conditions. In molecular flow, pumping action is produced by a fast moving flat surface dragging molecules in the direction of movement.
When viscous flow is approached, the simple momentum transfer does not work as well, because of increased backward flow due to the establishment of a pressure gradient rather than a molecular density gradient. As a result, the molecular drag stage may not achieve the desired pressure difference in viscous flow conditions.
Accordingly, there is a need for improved molecular drag stages for vacuum pumps.